The present invention relates to a screw tightener using coupled screws, and more particularly a mechanism for feeding a screw and reversely extracting coupled screws.
There is a screw tightener of such a type that a feed wheel is supported by a nose portion which is relatively movable to the screw tightener body in order to feed coupled screws supplied to the nose portion by having the rotation of the feed wheel interlocked with the movement of the nose portion to be forced into the screw tightener body. In a case where a worker uses such a screw tightener to screw a face material on a prime material on the ceiling, he would hold one side of the face material with one hand and push up the other side thereof with the tip of the screw tightener held by the other hand. Due to the load of the face material at this time, the nose portion may be forced slightly into the screw tightener body and allow the feed wheel to rotate according to a minute sign (the so-called "clap sign"), so that the screw is pushed out. If it is then attempted to drive in the screw at a predetermined position by separating the screw tightener from the face material, the feed wheel will rotate again when the nose portion is pressed against the face material, thus causing a new screw to be fed. For this reason, the screw initially fed may be wasted and besides the nose portion will be clogged with the two screws introduced into a narrow space.
Moreover, the screw will not sufficiently be driven in and the screw head will also be protruded from the face material if the indentation load applied to the nose portion is released before the screw is sufficiently driven in; consequently, it is required to tighten the screw again. However, the forced-in movement of the nose portion allows the feed wheel to rotate and causes a new screw to be fed. The screw thus needs tightening again after what has newly been fed is removed and there arise the problem of making troublesome the work of removing the screw that has already been fed and the problem of wasting screws.
In a screw tightener of the sort mentioned above, there arises the necessity of reversely extracting coupled screws that have been mounted once therein in such a case that the coupled screw needs replacing in the course of the screw-tightening operation, and a mechanism for reversely extracting coupled screws has been employed accordingly. As disclosed in Japanese Unexamined Patent Publication No. 100880/1990, a conventional mechanism for reversely extracting coupled screws has been contrived so as to reversely extract a screw by lowering an operative lever in a screw tightener.
However, the operative lever is known to have damaged a sheet of paper covering the surface of, for example, a plasterboard as a material to be screwed down because the lever projecting from the surface of the screw tightener may strike against the material. Moreover, such an operative lever has been disadvantageous in that it is inferior in not only operability but also design as it is left projecting from the screw tightener, thus giving a unfavorable impression.